Accessing WinRT from Desktop Apps (Part 2)

September 24, 2012

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In the previous post we’ve seen how to instantiate WinRT objects using the raw (Ro, pun intended) API. In this post, we’ll see some shortcuts to make our lives a little easier.

These shortcuts are part of the Windows Runtime template Library (WRL). This is a helper library, similar in spirit to the Active Template Library (ATL) used for classic COM work.

First we need to include the main WRL header, <wrl.h>. Also, we’ll include another header with some extra helpers, that are not included with the primary header:

#include <wrl.h>
#include <wrl/wrappers/corewrappers.h>

Next, we’ll add using namespace statements to make our lives a little easier:

using namespace Windows::Foundation;
using namespace Microsoft::WRL;
using namespace Microsoft::WRL::Wrappers;

Now for the real work in main().

To initialize WinRT on the current thread, we called RoInitialize() and at the end its counterpart, RoUninitialize(). A simple RAII class wraps these calls in the constructor/destructor, named RoInitializeWrapper:

RoInitializeWrapper init(RO_INIT_MULTITHREADED);

Next, we need an HSTRING that represents the Calendar’s full class name. Instead of calling WindowsCreateString etc., we can use the HString class wrapper provided by WRL:

HSTRING hClassName = HString::MakeReference(RuntimeClass_Windows_Globalization_Calendar).Get();

HString::MakeReference calls WindowsCreateStringReference, which is a way to create an “unmanaged” HSTRING, that does not need to be deleted because it points to memory that is already allocated in some other means; in this case, it’s a C-style string already defined in <windows.globalization.h> with the Calendar’s full class name.

To create the actual instance we’ll use another helper function, Windows::Foundation::ActivateInstance that accepts a class name (HSTRING) and returns an interface pointer to any supported interface. There’s no need to request IInspectable and then do a QueryInterface – the function takes care of that:

ComPtr<ICalendar> spCalendar;
HRESULT hr = ActivateInstance(hClassName, &spCalendar);

ComPtr<T> is WRL’s smart pointer (similar to CComPtr<T> from ATL).

That’s it. We can now use the Calendar in any way we want.

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